Observation of longitudinal quadrupole-mode oscillations of a bunch which were induced by RF phase modulation in the electron storage ring

It has been reported that the beam lifetime in electron storage rings can b e improved by applying a small phase modulation on the rf accelerating volt age at a frequency of two-times the synchrotron oscillation frequency. This effect can be explained as being due to the improved Touschek lifetime tha t is caused by an excited bunch-shape oscillation in the stored bunches. In order to understand this phenomenon further, we investigated the induced b unch-shape oscillation in the 2.5-GeV Photon Factory storage ring at High E nergy Accelerator Research Organization (KEK). Using both a streak camera a nd a spectrum analyzer, we observed the quadrupole-mode oscillations of a s tored single bunch very clearly. It showed a splitting of a single bunch in to three or two bunchlets, which depended on the modulation frequency. We r eport on these experimental results, and discuss them using computer simula tions and an analytical formalism.